Question 717437
Your proportion should set up like this: miles/hours or hours/miles. Either or, just make sure that both sides of the equation have the units in the same place (mi/hr=mi/hr not mi/hr=hr/mi)

210/3.5=x/6
Cross multiply
3.5x=1260
Divide both sides by 3.5 to get x alone
x=360

Make sure you include the units in your solution (360miles). 

To check the work, plug the solution back into the original equation and work it out.